When testing you PayPal integration on sandbox, you'll need to have sandbox user accounts set up to use to go through the payment flow.
Go to https://developer.paypal.com/developer/accounts/, log in using your PayPal account, and click on "Create Account", as below:
Enter in the accounts details for the new test user, including a unique email, account information, payment method, balance, etc, and click on "Create Account" at the bottom of the page once done. This will create the new account for you to begin using.
To see account details for this new user, expand the entry on the accounts page, and click on "Profile".
Once that profile information loads, clicking on the "Funding" tab will give you payment information for that account, including credit card information that may be used for direct credit card processing against sandbox.
NOTE: When using the sandbox API endpoints, you need to use sandbox test account to log in and pay for the test goods, as your live account information will not work.